This invention relates to the field of concentration measuring elecrodes and more particularly to an improved solid state electrode for use in potentiometric analyses and a novel method for the preparation thereof.
A variety of chemical indicators are available for measurement of the pH of aqueous media, but the most accurate, precise and reliable pH measurements are generally recognized to be those taken potentiometrically. In potentiometric analysis for pH, an electrometer, potentiometer or other voltage measuring device is utilized to measure the difference in potential between a special pH electrode immersed in the aqueous medium to be tested and a reference electrode that is in electrical communication with the same aqueous medium typically through a potassium chloride salt bridge. The pH electrode is one whose potential varies as a function of the hydrogen ion concentration of the aqueous medium, generally in accordance with the classical Nernst equation. The reference electrode is typically calomel or Ag/AgCl electrode, with the latter being preferred for physiological applications.
By suitable selection of measuring electrodes, concentration cells can be established for measurement of a variety of different ions. Potentiometric analysis can also be utilized for the determination of the equilibrium coefficients for various acid dissocation reactions or other ionic reactions in aqueous media.
For measurement of pH, it has long been conventional to use a glass electrode as the measuring electrode. The standard glass electrode includes a hollow bulb constructed of special ion selective glass, which serves as a membrane for charge transfer in response to the concentration of hydrogen ions in the medium to be measured. Contained within the glass bulb is a solution in which is immersed a standard electrode such as an Ag/AgCl electrode. A large variety of shapes and sizes of this basic electrode type, including micropipette, capillary, etc. have been fabricated. However, such "wet connection" electrodes are of limited utility in certain applications, including service under extremes of pressure and/or temperature and important physiological applications where small size is critical. In the present state of the art the minimum size attainable in a conventional wet connection glass electrode is a diameter about 1.5 mm for the bulb containing the internal electrode. Although micropipettes can be constructed in diameters smaller than this at their tips, they are unsuitable mechanically for muscular implant and intravascular use.
In the present state of technology, the glass bulb of a wet connection pH electrode must be made with a certain minimum thickness for adequate mechanical strength which, in combination with the relatively small area of the bulb, results in a relatively high impedance and increases both the response time of the electrode and the shielding required. Moreover, the response characteristic generally shifts with age due to alteration of the glass charge transfer properties through hydration by the internal solution. The wet connection pH electrode is also relatively fragile, difficult to reproduce consistently in small sizes by the necessary process of glass blowing, and subject to operation only within a relatively limited temperature range because of the generation of internal pressure.
In order to overcome some of the difficulties associated with conventional wet connection glass electrodes, proposals have been made for the use of solid state electrode for pH measurement. Such were first proposed by M. R. Thompson, Journal of Research National Bureau of Standards, Vol. 9 p. 833 (1932) and pioneered by Friedman et al. Proceedings of The Society of Experimental Biological Medicine Vol. 99 p. 727 (1958). However, successful work with solid state electrodes has been largely limited to capillary (flow through) electrodes, wherein metal is applied to the outside of a glass tube through which the medium to be measured is caused to flow. The art has encounterd considerable difficulty in developing a feasible method for the production of dip-type solid state glass electrodes, in which a metal conductor is contained within an ion-selective glass envelope or membrane.
Recent developments in solid state glass electrodes are described in Metz et al. U.S. Pat. Nos. 3,498,901 and Szonntagh 4,031,606. Metz et al. describe a glass electrode having a superficially oxidized copper connection with a thin overlayer of glass that is fused to the oxidized surface through partial diffusion of the oxide into the glass. Szonntagh describes a laminate electrode having stacked layer of glass, silver chloride and silver encapsulated in a potting compound. From his patent disclosure, it would not appear that the Szonntagh electrode is adapted for production in such dimensions and configurations as to be suitable for critical physiological uses.